This invention relates to the recovery of nickel and cobalt from ammoniacal solutions containing these metals and, in particular, to the recovery of nickel and cobalt from such solutions by solvent extraction techniques.
Numerous nickel and cobalt-enriched ammoniacal solutions are known to those skilled in the art. For example, ammoniacal carbonate solutions containing nickel and cobalt are produced commercially by heating cobalt-containing nickeliferous ores in a furnace with a reductant gas and then leaching the reduced ores in the presence of oxygen with an aqueous solution of ammonium hydroxide and ammonium carbonate. Examples of the production of such solutions are described in U.S. Pat. Nos. 1,487,145 and 3,100,700, and in "The Winning of Nickel," J. R. Boldt, Jr., Van Nostrand Co., Inc., Princeton, N.J., 1967, pp. 425-537.
Ammoniacal chloride solutions containing nickel and cobalt are prepared by leaching reduced lateritic ores with an aqueous leach liquor containing ammonium hydroxide and ammonium chloride. Ammoniacal chloride solutions are also prepared by dissolving or redissolving a nickel and cobalt-containing material such as a nickel matte, a basic nickel carbonate, mixtures of nickel carbonates and cobalt carbonates, a nickel oxide, nickel scrap, a nickel alloy or mixtures of nickel sulfide and cobalt sulfide containing nickel and cobalt in varying proportions, in an aqueous solution of ammonium hydroxide and ammonium chloride, or in aqueous hydrochloric acid followed by adjustment of the solution pH to above 7 and normally above 8, with ammonia.
Ammoniacal nitrate solutions containing nickel and cobalt are prepared by leaching reduced lateritic ores with an aqueous leach liquor containing ammonium hydroxide and ammonium nitrate. Ammoniacal nitrate solutions are also prepared by dissolving or redissolving a nickel and cobalt-containing material such as nickel matte, a basic nickel carbonate, mixtures of nickel carbonates and cobalt carbonates, a nickel oxide, a nickel alloy, nickel scrap, mixtures of nickel sulfide and cobalt sulfide containing nickel and cobalt in varying proportions, in an aqueous solution of ammonium hydroxide and ammonium nitrate, or in aqueous nitric acid followed by adjustment of the solution pH to above 7, and normally above 8, with ammonia.
Ammoniacal sulfate solutions containing nickel and cobalt are prepared by leaching reduced lateritic ores with an aqueous leach liquor containing ammonium hydroxide and ammonium sulfate. Ammoniacal sulfate solutions are also prepared by dissolving or redissolving a nickel and cobalt-containing material such as a nickel matte, a basic nickel carbonate, mixtures of nickel carbonates and cobalt carbonates, a nickel oxide, a nickel alloy, nickel scrap, mixtures of nickel sulfide and cobalt sulfide containing nickel and cobalt in varying proportions, in an aqueous solution of ammonium hydroxide and ammonium sulfate, or in aqueous sulfuric acid followed by adjustment of the solution pH to above 7, and normally above 8, with ammonia.
Numerous techniques have been developed to separate and recover the nickel and cobalt values from these solutions in a convenient form. One such technique involves the extraction of the solutions with various oxime and oxine compounds (of the type disclosed, for example, in U.S. Pat. Nos. 3,276,863, 3,592,775, 3,655,347 and 3,725,046) dissolved in an organic diluent in order to selectively extract the nickel into the organic phase and leave the cobalt in the aqueous ammoniacal phase.
The application of this technique specifically to the recovery of nickel and cobalt from ammoniacal solutions is described for the case of oxime compounds in U.S. Pat. No. 3,276,863. In this patent, a synthetic ammoniacal solution containing nickel and cobalt is contacted with air or another oxidizing agent and then extracted with the oxime extractant which selectively extracts the nickel. The nickel-loaded extract is stripped with a solution of a strong acid or ammonia, or other aqueous solutions capable of causing the nickel to transfer out of the extract, to recover the nickel and regenerate the organic extractant.
One aspect of the present invention involves an improvement of the process described in U.S. Pat. 3,276,863 applicable when the nickel-loaded extractant is stripped with aqueous sulfuric acid. Such may be the case, for example, when an object of the process is the manufacture of an electrolytic solution suitable for the production of high purity electrolytic nickel. When sulfuric acid is used as the stripping agent, several problems can arise in the solvent extraction technique described in U.S. Pat. No. 3,276,863. For example, during extraction with the oxime extractant some ammonia is transferred from the aqueous ammoniacal leach solution to the organic phase. Even though most of the ammonia remains in the aqueous phase during extraction and is eventually recovered for reuse in the process, the small amounts that do transfer to the organic phase represent a loss of ammonia since they normally are not recovered. More importantly, these small amounts of ammonia can react with the sulfuric acid in the stripping operation, and by neutralizing some of it, cause the acid to lose some of its stripping power. The ammonia also frequently forms compounds which can interfere with subsequent processing of the product strip solution. For example, the presence of ammonia in a nickel-loaded extract can cause the formation of compounds such as (NH.sub.4).sub.2 SO.sub.4 and (NiNH.sub.4).sub.2 (SO.sub.4).sub.3 in the nickel-loaded strip solution when the extract is stripped with sulfuric acid. These compounds not only consume some of the stripping agent, but their presence can also interfere with the subsequent electrolysis of the product strip solution. The double salt (NiNH.sub.4).sub.2 (SO.sub.4).sub.3, for example, precipitates and is deposited with the metallic nickel causing a purity problem and a decrease in the amount of metallic nickel recovered in the process.
A second aspect of the invention overcomes other problems associated with the process described in U.S. Pat. 3,276,863 which arise from the transfer of cobalt into the organic extractant during the extraction step. This cobalt tends to eventually load the recycled organic extractant with cobalt after a few passes in the circuit. This is generally known in the art as "cobalt poisoning of the organic". This "poisoning" presents a problem because for each gram of cobalt present in the recycled organic extractant, an equivalent one gram of nickel in the feed ammoniacal leach solution remains unextracted. Thus to maintain a practical extraction efficiency, the cobalt-poisoned organic must be bled from the system in order to maintain a low cobalt concentration in the recycled extractant. The bleeding of poisoned extractant represents not only a loss of valuable cobalt and expensive extractant, but also creates a waste disposal problem since the extractant may not be sent to waste with other waste streams because of pollution considerations and therefore has to be impounded.
It is a general object of the present invention to provide methods for overcoming the above mentioned problems associated with prior art solvent extraction processes for recovering nickel from ammoniacal solutions containing nickel and cobalt.
It is a more specific object of the invention to provide a method for removing ammonia from the extractant used in these prior art solvent extraction processes, in cases where the nickel is stripped from the extractant by aqueous sulfuric acid, before the stripping operation takes place so that the ammonia is not available to react with the acid to create an ammonia loss and unwanted contaminants in the acid strip solution.
It is another object of the invention to provide in such prior art solvent extraction processes a method for reducing the sulfate impurities normally found in the nickel-loaded strip solution in the case where the extractant is stripped with aqueous sulfuric acid, so that the strip solution becomes more suitable for use in the production of high purity electrolytic nickel.
It is another specific object of the invention to provide in such prior art solvent extraction processes a method for removing from the extractant a sufficient quantity of the cobalt transferred to the extractant during the extraction step to eliminate "cobalt poisoning" of the extractant, so that the extractant can be recycled to the process.
It is another object of the invention to provide in such prior art solvent extraction processes a method for eliminating the need for discarding cobalt enriched organic extractant from the process in order to prevent "cobalt poisoning" of the extractant and to thereby avoid the waste disposal problem associated with discarding the extractant, and the loss of valuable cobalt and extractant.
It is another object of this invention to provide in such prior art solvent extraction processes a method for recovering, for recycle to the process, the ammonia and cobalt transferred to the extractant during the extraction step.
These and other objects of the invention will be apparent to those skilled in the art from a consideration of this entire disclosure.